The present invention relates to a pile guide, particularly, but not exclusively, to a pile guide for underwater pile driving, e.g. for stabbing poles directly into the seabed.
It is known to provide a guide for aligning a pile with the surface of a substrate into which the pile is to be driven and to provide stability for a piling hammer. One such guide is described in the present applicant""s International application published under WO99/11872 (U.S. Pat. No. 6,354,767 B1). As shown in FIG. 1, the guide is supported on a base frame (10), which has a substantially rectangular footprint. The base frame is made up of a welded framework of girders (12) to which mud mats (14) are attached to spread the load (weighing about 35 tonnes) across the surface of the substrate (e.g. sea bed). Even so, when deployed on soft soils e.g. silt, there is a risk that the guide may sink into the substrate, either under its own weight or during pile driving. One way of alleviating even obviating this risk is to increase the basal area of the base frame to spread the load still further.
Difficulties have been encountered in controlling the deployment of large objects such as pile guides at sea due to hydrodynamic forces generated by wave-induced movement at the water surface. In many cases, the more water which can become xe2x80x9ctrappedxe2x80x9d above the object during deployment, the greater the hydrodynamic forces might be. For this reason, objects with large flat surfaces (e.g. the base frame of a pile guide) are often deployed at an angle to encourage water to run off them rather than become trapped, thereby minimising potential hydrodynamic forces at work. Even so, certain large objects may only be deployed in relatively calm seas to ensure that hydrodynamic forces acting on the objects do not exceed safe, threshold levels.
The present applicant has appreciated the desirability of a novel pile guide which is more stable in soft soils than conventional pile guides, and yet which is at least as readily deployed at sea as existing pile guides.
In accordance with one aspect of the present invention, there is provided a pile guide for supporting a pile as it is driven into a substrate, comprising a base frame and a pile guide member mounted on the base frame, wherein the base frame comprises at least one substrate-engaging support, moveable between an inoperative position and an operative position, with movement of the at least one substrate-engaging support into the operative position resulting in an increase in area of an active surface of the base frame which, in use, rests on the substrate.
The at least one substrate-engaging support may be mounted on the pile guide for movement relative thereto and, also, may be attached to the base frame via a respective pivot.
The present applicant has found that the potential difficulties of deploying pile guides at sea, and the potential problems of supporting pile guides on soft soils may both be alleviated by using the moveable substrate-engaging support. When in the inoperative position, the substrate-engaging support may not contribute significantly to the xe2x80x9cfootprintxe2x80x9d or substrate-engaging area of the base frame, which is clearly advantageous as it should not unduly increase hydrodynamic forces acting on the pile guide during deployment at sea. When in the operative position, the substrate-engaging support may contribute significantly to the xe2x80x9cfootprintxe2x80x9d of the base frame, helping to resist any tendency to sink in soft soils.
The base frame may further comprise a hub portion to which the at least one substrate-engaging support is mounted. The at least one substrate-engaging support may be pivotally connected to the hub portion for pivotal movement around a respective pivot axis. The at least one pivot axis may be aligned substantially parallel to the base plane of the base frame.
A locking mechanism may be provided to lock the at least one substrate-engaging support when moved into the operative position. The locking mechanism may comprise a bolting arrangement comprising a bolt and at least one aperture engagable by the bolt. For example, a bolt may engage a pair of apertures, one associated with a respective substrate-engaging support, the other associated with the hub portion, the apertures being in registration when the respective substrate-engaging support is in the operative position. The bolting arrangement may be activated by a diver or by remote control.
The at least one substrate-engaging support may be biased by a biasing arrangement (e.g. a counterweighting arrangement) to move into the operative position when released from the inoperative position. In this way it is possible for the pile guide to be launched in a retracted form and subsequently to be reconfigured in an extended form. For example, during deployment, the at least one substrate-engaging support may be restrained by a restraining device attached to the pile guide, e.g. tethered to the pile guide member, to restrain it in the inoperative position. As soon as the tether is released (e.g. cut), the substrate-engaging support is urged (e.g. under gravity) to the operative position.
Alternatively, or in addition, an actuator may be provided to drive the at least one substrate-engaging support from the inoperative position to the operative position. The actuator may comprise a hydraulic ram, and may also hold the at least one substrate-engaging support in the operative position, thereby preventing movement back towards the inoperative position.
The pile guide may comprise a plurality (e.g. four) substrate-engaging supports symmetrically disposed around the hub portion.
The substrate-engaging supports may increase the substrate-engaging area of the base frame by at least 10%, perhaps at least 25%, when moved from the inoperative position to the operative position. Furthermore, the substrate-engaging supports may increase the substrate-engaging area of the base frame by at least 50%, perhaps even at least 100%, when moved from the inoperative positions to the operative position.
In accordance with another aspect of the present invention, there is provided a method of deploying a pile guide at sea, comprising the steps of:
providing a pile guide comprising a base frame and a pile guide member mounted on the base frame, the base frame comprising at least one substrate-engaging support moveable between an inoperative position and an operative position;
lowering the pile guide into the sea through its splash zone with the at least one substrate-engaging support in the inoperative position so that the substrate-engaging area of the base frame is at a minimum; and
moving the at least one substrate-engaging support into the operative position to increase the substrate-engaging area of the base frame when the pile guide has been lowered through the sea splash zone.
The method may further comprise: lowering the pile guide until it is adjacent, i.e. a relatively short distance from (e.g. within 10 m) and directly above the substrate on which it will rest before moving the at least one substrate-engaging support into the operative position.
Furthermore, while lowering the pile guide through the sea splash zone, the pile guide may be orientated with its largest planar surface inclined to the sea level.